Hydrogen for use in fuel cells (and possibly as a fuel in internal combustion engines) appears poised potentially as the next major evolution in energy usage. Unfortunately, safely storing hydrogen and other low molecular weight fuels is currently very difficult and expensive. Such fuels are now stored in pressurized tanks or in hydride storage systems. Hydride storage is far safer than compressed hydrogen gas storage, and safer even than gasoline on an equivalent-energy basis (Reilly, J. J. Sci. Amer. 1980, 242(2), 2). However, hydrides carry both weight and cost penalties versus compressed hydrogen. As another alternative, hydrogen can be created as needed by reformers, but efficiencies are reduced. Hydrogen vehicle systems' space and weight penalties versus gasoline are about 5 times the space if stored as compressed gas, or still 3 times the space plus between 40 to 470 extra pounds of weight if stored as a metal hydride, even after taking into account the greater efficiency of hydrogen fuel cells (HFCs) versus gasoline internal combustion engines (ICEs). Additionally, these space limitations require storage to be closer to passengers, compounding safety concerns.
These storage limitations penalize not just on-board vehicle storage and vehicle range, but also the capacity for overall transportation and distribution of hydrogen versus gasoline or other existing fuels, and its storage prior to use. These limitations in turn limit where, how efficiently, and how cleanly hydrogen can be produced. For example, hydrogen generation onboard vehicles from methanol or gasoline reformers cuts total emissions only 7–35%, while steam reforming at service stations would reduce emissions by 40%, and remote generation would reduce emissions by 60–70%, given a practical and economical storage method. A better method of both on-board storing, and transporting and distributing, of hydrogen, including pure HFC vehicles would have a significant and broad positive impact on this emerging new industry.
Generation of hydrogen at the site of remote electric power generating plants and then shipment of the hydrogen to markets using economical storage systems could entail less energy loss than if electricity is transmitted to local service stations for generating hydrogen on site. Cheaper natural gas costs could also be accessed by generating hydrogen closer to the sources of gas. Economies of scale of larger volume hydrogen production could be realized. Transmission-distribution wires in many cases could be avoided. And attendant air pollution could be removed from the local area urban centers, where smog is the principal concern. Renewable energy supplies (solar, wind, etc.) could also be accessed given an economic transportation system.
Stationary HFCs are likely to be introduced in homes and industry concurrently with vehicular use. Because manufacturing costs of HFCs and hydrogen generators will both decline with the volume permitted by introduction of HFCs to the vehicle market, HFCs should simultaneously begin penetrating homes and industry, for both heating and electricity. Whereas the proton exchange membrane (PEM) HFC is targeted for vehicles, with an efficiency of about 50% versus 25% for the ICE vehicle, still higher-efficiency fuel cells are being developed for stationary use, with efficiencies of 60–80%, versus 30–60% for conventional power generation (the upper ends of ranges in both cases including cogeneration). These stationary fuel cells include the molten carbonate, phosphoric acid, solid oxide, alkaline, direct methanol, and other fuel cells.
HFCs for portable power products, such as, but not limited to computers, can be introduced concurrently with vehicular use; again, riding the cost and technology curves developed for HFCs and hydrogen generation for the other markets. These stationary HFC markets, portable power product markets, and vehicle HFCs will all require hydrogen to be safely, inexpensively, and conveniently delivered, as well as stored before use.
HFC's main problems are vehicle range, safety, and hydrogen fuel availability. These problems are all in turn aspects of the problem of hydrogen storage. A HFC system may achieve the same range as for gasoline ICE (a usual target being 380 miles), but 3–5 times the space and possibly far greater weight are required compared to gasoline. The extra space required also adds to real or perceived safety concerns. These space and weight penalties also affect the ease of transportation and distribution of hydrogen, which in turn makes vehicle range concerns still more sensitive. The space and weight limits, and associated safety concerns, represent the largest negatives for HFCs.
Most critical is the space required for compressed hydrogen storage. Hydrogen today is stored at 2,000–2,500 psi (14–17 mPa) and requires large, heavy containers. Even at 5,000 psi, the standard currently being pursued for development in the auto industry, the space required is still about 5 times that of gasoline, even after allowing for the greater efficiency of the HFC versus the ICE. Table 1 summarizes these limits, along with weight limitations of the various systems.
This greater space requirement either limits the vehicle's range between refueling—especially important with a limited hydrogen infrastructure—and/or requires storage closer to passengers, raising safety concerns.
TABLE 1Space and Weight of Fuel and Tanksfor 380 Mile/Tank Range Vehicle5000 PsiGasolineHydrogenHydrideMileage29 mpg64 mpg64 mpg(2.2 times(2.2 timesgasoline ICE)gasoline ICE)Gallons13 gallons4.7 kilograms4.7 kilogramsTank Size14 gallonsVolume1.87 cubic ft.9.36 cubic ft.5.7 cubic ft.Volume versus—5 times3 timesGasolineWeight Fuel73 lbs.10.3 lbs.10.3 lbs.Weight Tank29 lbs.100 lbs.140–570 lbs.*Total Weight102 lbs.110 lbs.150–580 lbs.*Weight versus—Similar+40–470 lbs.*Gasoline*Lower weight is magnesium hydride; higher weight is more economical and currently practical iron-titanium hydride.Source: “Onboard Compressed Hydrogen Storage,” by Brian James, C. E. Thomas, and Franklin D. Lomax, Jr., Directed Technologies, Inc, Arlington, Va., February, 1999.
If hydrogen gas were compressed to 10,000 psi (also being considered for development), it would occupy about the same space as liquid hydrogen; but this is still about 3 times that of gasoline. However, even pressurizing to 5,000 psi may raise some significant safety concerns, and pressuring to 10,000 psi, when and if economically possible, might simply add to those concerns. Liquid cryogenic hydrogen storage requires very low temperature of −253° C.
Liquid cryogenic hydrogen, which takes up 3 times the space than gasoline, is impractical due to the extremely low temperatures required (minus 423 degrees Fahrenheit), the energy and cost that must be expended to liquefy hydrogen (approximately doubling its delivered cost), and the losses during storage as the liquid hydrogen slowly boils off and escapes. Some such losses might also occur in 10,000 psi compressed hydrogen gas.
Hydrogen has many safety concerns. Hydrogen must in any case be compressed to a significant pressure, since uncompressed hydrogen (i.e., at atmospheric pressure) has only 1/1330 the energy density, and thus takes 1330 times the space, of gasoline. The onboard vehicular storage of hydrogen gas at any pressure raises safety concerns in the event of an accident, since the storage tank must be far stronger than that for gasoline to prevent rupture. The sudden release of such highly compressed gas could itself pose a significant safety hazard in the event of an accident, spewing an instantly flammable cloud.
Slower leaks likewise pose an ongoing concern. This is especially true since parking indoors would create its own safety problems, which would require re-engineering buildings. This is because hydrogen rises, and most garages are not protected from upward rising gasses. Many bedrooms and most living spaces are built over the garage in current housing designs. The slow boiloff of very high pressure 10,000 psi gas (mentioned above) could be hazardous.
To accommodate the extra required space, compressed hydrogen fuel tanks may have to extend either under the vehicle floorboards or in overhead roof areas, placing the fuel closer to passengers. Locating hydrogen tanks closer to passengers appears even riskier than for gasoline tanks.
Carbon fiber-reinforced plastic compressed hydrogen fuel tanks will improve. However the few occasional inevitable accidents (including some that have already occurred) could cause continuing safety concerns for compressed natural gas onboard vehicles under extreme pressures, especially since hydrogen already has a (largely undeserved) safety image problem.
To counter the need for onboard storage of hydrogen and thus eliminate altogether such safety concerns, and perhaps even more importantly, to provide greater availability of hydrogen fuel supplies during early years of introduction, auto companies are considering alternatives to pure HFCs which have onboard hydrogen reformers. These onboard reformers manufacture the hydrogen only as it is actually used in the fuel cell, thus eliminating storage and safety concerns. These reformers typically use methanol or gasoline fuel, thus reducing or eliminating fuel availability concerns. However, these reformers are inferior to pure hydrogen HFCs by virtually all other measures: emissions reductions, cost of fuel per mile, cost of vehicle, and oil import reductions.
Hydrogen storage as a hydride virtually eliminates the safety penalty as well as much of the space penalty of compressed gas storage versus gasoline; however, the tradeoff is weight and cost penalties. Stored as a compressed gas, hydrogen is on parity in weight with gasoline, as shown in Table 1. Hydrogen on an energy basis weighs only one third of gasoline. This reduces to about one seventh the weight of gasoline required in a vehicle after allowing for the estimated 2.2 times greater efficiency of the HFC versus the ICE vehicle. However, with the extra weight of the tank, compressed hydrogen storage still weighs about the same (Table 1). Light-weight 5,000 psi carbon fiber-wrapped compressed gas storage systems under development, which will store up to 10% of hydrogen by weight, will make this approximate weight parity possible.
Thus, as Table 1 shows, a 14 gallon gasoline tank, holding 13 usable gallons and capable of traveling 380 miles at 29 miles per gallon, would weigh about 102 pounds: 73 pounds of gasoline and 29 pounds of tank and related equipment. A 4.7 kg. compressed hydrogen storage system capable of the same range would weigh about the same, 110 pounds: including 10.3 pounds of hydrogen and 100 pounds of tank and related equipment. In this compressed gas form the limitation is not weight, but rather the very significant 5 times greater space, and accompanying safety concerns.
Hydrogen also faces transportation, distribution and storage problems prior to use. Transportation via tank truck of either liquid hydrogen or highly compressed (up to 10,000 psi) hydrogen is possible. However, trucking of liquid hydrogen would not be economic for widespread HFC vehicle use due to the higher costs of liquefaction, and trucking of compressed hydrogen gas would, as of now, raise problems of both safety and economic cost. Pipelines are prone to hydrogen embrittlement, and in any case, a network does not now exist for distribution of hydrogen either to service stations or to homes or industry.
The answer now being considered is on-site service station generation of hydrogen, either by steam methane reforming or by electrolysis. However, electrolysis is only an initial, interim solution, and on-site steam methane reforming adds to the local-area smog pollution, reducing some of the smog-abating benefits of HFCs.
Remote-site steam methane reformers would reduce the local-area smog pollution of HFCs to virtually zero, making them more substantial contributors to clean air in urban centers, where smog is the major concern. Remotely sited steam methane reforms could also be larger, giving greater economies of scale and cheaper costs of manufacturing hydrogen—if an economical means existed to transport such hydrogen to the point of use.
If steam methane generation or electrolysis on site at the service station is used, a practical means of hydrogen storage would enable these reformers or electrolyzers to operate “steady state,” including at night when the service station is closed. This method appears more economic than the other choice to “follow load” of traffic flow.
Hydrogen can be reformed as needed from other chemicals (e.g., methanol, gasoline, or hydrocarbons) that are easier to store than hydrogen and can take advantage of existing distribution systems. However, inefficiencies, poisons, and environmental penalties partly offset these benefits.
Natural gas and other light-density fuels face the same storage problems as hydrogen, and could likewise benefit from a more practical means of storage.
Hydrogen storage methods considered above include physical storage in a compressed gas or liquefied state, and solid-state storage using gas-on-solid adsorption in materials such as, but not limited to high surface area carbon.
Activated carbon or activated charcoal is usually used for gas-on-solids adsorption. This technology works better at low temperatures. The equipment and cost of maintaining low temperatures complicate use of this technology, especially in vehicles.
Solid-state storage, gas-on-solids and metal hydrides are options which are safer technologies and they provide high storage capacity than physical storage systems. They are more expensive and heavier. Current research is aiming to determine the hydrogen adsorption/desorption properties of commercially available carbons and zeolites. Solid state storage capacities, rates of charge and discharge, thermal and mechanical effects and costs of available materials are important cost and operating parameters for hydrogen storage systems.
With gas-on-solids adsorption technology, hydrogen can be stored by being adsorbed onto the surface of activated carbon. This technology provides better volume density than compressed gas storage. The weight and volume densities of this application are comparable to liquid hydrogen systems. In a main drawback, the adsorption of hydrogen on carbon requires maintaining a temperature below 150 K (−190° F.).
A metal hydride form of hydrogen storage would reduce hydrogen's space penalty versus gasoline from fivefold to about threefold, even before considering greater vehicle design efficiencies. Even more important, storage as a metal hydride would virtually eliminate safety concerns of hydrogen storage, to even less than for gasoline. This greatly increased safety would in turn permit storage closer to passengers, thus for practical purposes in totally new vehicle designs perhaps eliminating the space limitation altogether. Seen in this light, safety concerns may be the principal reason for space limitations of compressed gas storage.
However, metal hydride systems would add 40–470 pounds more weight than gasoline (on a total system basis), depending on which of a wide range of hydride metals is used. The cost of these metals may also be significantly greater than for a carbon fiber wrapped compressed hydrogen storage tank. These weight and price penalties of hydrides have, unfortunately, more than offset their safety and volume benefits in auto engineering thinking to date.
With metal hydride technology, certain metals, alloys and other materials can be used to absorb and retain hydrogen under specific temperature and pressure conditions. They release hydrogen under different conditions. These metals are called metal hydrides when containing hydrogen. Magnesium hydrides are popular because magnesium is a relatively cheap and abundant metal and can absorb large amounts of hydrogen for its weight. Hydrides are safe and a have very high hydrogen volumetric storage capacity as compared with other methods of hydrogen storage. Unfortunately, metal hydrides are currently expensive. Currently, lower cost hydride materials require high temperatures to release hydrogen. On the other hand, hydrides which release hydrogen at lower temperatures are expensive and have less storage capacity.
A typical hydride storage system can contain several forms hydrogen at its different charging, storage, and discharging stages. A solid solution of hydrogen atoms can exist in a metal lattice or coexist with the monohydride phase of the hydride (e.g., XH, where X is a hydride-forming metal or other element). A monohydride phase can exist alone. Both monohydride phase and dihydride phases (e.g., XH2) can coexist. A dihydride phase can exist alone. See, E. Wiberg and E. Amberger, Hydrides of the Elements of Main Groups I-IV, Elsevier, 1971, pp. 1–12. These storage mechanisms are usually different at a hydrogen-binding material's surface and in its bulk.
Hydrogen reacts with many elements to form compounds. Of these, the transition metals (Groups IIIA through VIIIA in the periodic table, including the lanthanides and actinides) are most important because they can absorb large quantities of hydrogen and form metallic hydrides. Metallic hydrides exhibit the general properties of metals, i.e., high electrical and thermal conductivity, hardness, and metallic luster. Typically hydrides are powders with average diameters of a few microns.
There are many hydride-forming materials. The terms hydride materials and hydride-forming materials are used interchangeably in this application and in many references. New alloys and other materials will allow even better hydride properties. Currently popular metal hydride systems include alloy ratios of a first metal A, and a second metal B to form AB5 (e.g., LaNi5), AB (e.g., FeTi), A2B (e.g., Mg2Ni), and AB2 (e.g., ZrV2). A few examples of currently used hydrides include: LaNi4.7Al0.3, Ti0.98Zr0.02V0.45Fe0.1Cr0.05Mn1.4, Ca0.2M0.8Ni5 (wherein M represents mischmetal), CaNi5, Ni64Zr36, Fe0.8Ni0.2Ti, FeTi, Fe0.9Mn0.1Ti, CaNi5, LaNi5, LaNi4.7Al0.3, Mg2Ni; Mg2Cu, Mg, V, Ti, Zr, Th, Pd, Ca, and Li. A small percentage of another metal can be added to the alloy to affect performance. Other hydrides include non-reversible chemical hydrides such as, but not limited to LiHx, AlH, NaH, and B2H4. Liquid organic hydrides include chemicals such as, but not limited to decaline, and methyl cyclohexane used with a catalyst at 200° C. Mischmetal is an alloy of rare earth metals containing about 50% lanthanum, neodymium, and similar elements.
The equilibrium pressure-composition-temperature relationships of a metal/hydrogen system can be conveniently summarized by a P-C isotherm of which an idealized version is showed in FIG. 1. Hydrogen gas pressure is plotted versus the ratio of hydride:metal. The label C can be the concentration of hydrogen or the ratio of hydrogen to metal. The P-C curve shows three distinct sections. Initially the isotherm ascends fast (section A–B) as hydrogen enters the metal lattice and occupies interstitial positions. At low concentrations of hydrogen, the composition/pressure relationship is ideal and obeys Sievert's Law:H/M=KsP1/2  (Equation 1)
where H/M is the hydrogen to metal ratio, Ks is Sievert's constant, and P is the equilibrium hydrogen pressure. As the hydrogen content in the metal increases, the hydrogen atoms interact (via the elastic strains introduced in the metal lattice) and the pressure/composition behavior departs from this ideality. This is reflected by a decrease in the slope of the isotherm. At a critical average hydrogen concentration, the metal/hydrogen system forms a new hydride phase (see area C1 in FIG. 1). There is a discontinuity in an increasing amount of hydrogen that the metal can store. The flat, or plateau region in the P-C isotherm corresponds to the co-existence of the metal and hydride phases. To force more hydrogen into the alloy requires increasing the external gas pressure. This is represented by the rapid increase in the C-D region of the P-C isotherm curve.
In general, the plateau pressure for hydrogen loading is different from that for unloading. This pressure difference is called hysteresis. A flat plateau in the P-C curves is usually a required feature for gaseous hydrogen delivery systems to allow a large quantity of hydrogen can be stored reversibly at a constant pressure. It is important that the plateau be as wide as possible (large hydrogen storage capacity) and that at room temperature the plateau pressure be close to atmospheric pressure, since the storage container does not need to be especially strong and can thus be of light weight. Equation 1 may be affected or followed more closely by using nanoparticular hydrides instead of bulk hydrides.
The palladium/hydrogen system has been studied extensively, beginning with the early work of Graham well over a hundred years ago. Palladium is an attractive material due to its ability to readily dissociate molecular hydrogen to atomic hydrogen at its surface, but is overly expensive. Unfortunately, the direct replacement of palladium for cheaper metals or alloys is hindered because these metals form oxide or other layers and the reduced surface reaction limits the hydrogen flux into the metal.
To exploit the rapid bulk diffusion of hydrogen in the refractory metals, a palladium can be coated on a less expensive material. This allows the dissociation of the molecular hydrogen by the surface palladium layer, transport through the refractory metal bulk, and finally reassociation on the opposite surface.
In one example, the Group V metals are subject to embrittlement. However, the regime where this is a problem is well below room temperature. Should the surface palladium layer develop defects, this would not render the material useless since it would merely expose a small area of the refractory metal. Composite metal materials, such as, but not limited to plating foils of Group V metals (e.g., vanadium) with thin layers of palladium, are known. While it is clear that viable composite metal materials in larger scales have been constructed, improvements of the process are still required to make these structures more efficient. Removal of the refractory metal surface oxide layer is important to ensure hydrogen flux into the metal. Various chemical and mechanical techniques have been used to achieve this, but most allow regrowth before coating with the top palladium or palladium alloy. Another concern is the quality of the palladium or alloy layer.
The hydriding reaction proceeds inwardly from the surface of the alloy (Reilly, J.; J. Sci. Amer. 242(2), 2, 6 (1980)). Cracks and fissures are created, increasing the surface area. However, this surface area is probably still much lower than present in nanophase materials. Porous hydrides are described by Congdon in U.S. Pat. No. 5,443,616.
In the past several years nanophase materials, including metals and ceramics, have begun being designed and manufactured. These have dramatically different characteristics than their precursor (parent) materials, making them act almost like new materials. These different characteristics can be customized by controlling the size, as well as shape and crystalline character of the nanophase grains. For example, see Richard W. Siegel's review in “Creating Nanophase Materials,” Scientific American, December, 74–79 (1996).
Nanoparticles include atomic clusters, molecular clusters, agglomerates, micelles, and other particles with nanometer dimensions. Nanoparticles are usually made by such techniques as chemical vapor deposition (CVD), physical vapor deposition (PVD), physical vapor synthesis (PVS), reactive sputtering, electrodeposition, laser pyrolysis, laser ablation, spray conversion, mechanical alloying, and sol gel techniques. New, lower cost methods are being evolved from these and similar means of production. In general, nanoparticles can be synthesized from atomic or molecular precursors or by chemical or physical means.
Many materials with some nanoscale feature (e.g., crystals or grains) are often confusingly called nanoparticles, whereas in fact they are larger composite structures with some nanoparticle features. These large structures should be called nanostructured materials, nanostructures, or nanomaterials. For example, Ovshinsky describes hydrides made of nanocrystallites in U.S. Pat. No. 5,840,440. However, these nanocrystallites only describe the size of the crystals making up the continuous hydride bulk structure. These crystallites integrally form the bulk material. There are no separable nanoparticles in these hydrides.
Two other examples use mechanical alloying by ball milling. Alloying nanoscale particles by ball milling was studied by Holtz and Imam [J. Mater. Sci., 32 (1997) 2267]. However, the resulting product was pressed into large pellets. Song describes agglomerates with isolated grains of submicron Mg2Ni hydrides created by ball milling [Song, Y. M., Int. J. Hydrogen Energy, 20(3) 221–227]. However, it appears that most of the isolated grains are components of larger particles or agglomerates as depicted in his electron micrographs. Moreover, size of the particles is measured by lower resolution scanning electron microscopy, which depicts a macrostructure with small microfeatures, and Song does not differentiate between agglomerates, grains, and individual particles.
Singh, et. al. [J. Alloys and Compounds, 227 (1995) 63–68] describes more extensive ball milling with smaller primary particles. However, his TEM shows that the majority of these small primary particles are combined into parts of larger aggregates. Due to the high energy imparted by mechanical alloying, the nanoparticles are fused together into larger alloyed aggregates. Such extensive ball milling is expensive.
Imamura, et al. [J. Less-Common Metals, 135 (1987) 277] describe making small hydride particles by evaporating magnesium into an atmosphere containing THF. This method can theoretically engineer a separation of the nanoparticle metals, but is not clearly demonstrated to do so in this instance. Imamura only roughly estimates particle size based on surface areas calculated by BET surface analysis. This assumes perfect spherically shaped particles with no surface roughness. Imamura did not know the number of such particles in the sample, their packing, their shapes, agglomerations, or their size distribution. Methods of estimating average particle size distribution are not amenable to particles this small because of numerous measurement problems. BET can be used to measure pore sizes due to their surface area, but cannot measure particle sizes of loose powders. For these reasons BET is especially not suited for measuring nanoparticles due to numerous measurement problems. Again, grain sizes or solvated solids within a THF medium are described instead of discreet nanoparticles. This THF work describes metal particles held together by THF to form THF-impregnated aggregates or wet agglomerates by an expensive vacuum process. The primary particles are not significantly separable into discrete nanoparticles.
Carbon or graphite nanotubes are a nanomaterial that has been investigated for hydrogen storage. Most nanotubes have a submicron wall thickness and sometimes a diameter. Critical dimension is the smallest dimension of an object. The length of nanotubes is usually approximately a micrometer (micron). The smallest nanotube diameters are still much larger than interstitial sites in hydrides.
Carbon nanotubes trap hydrogen within the inside diameter of the nanotube or as gas-on-solid adsorption. In a big difference, hydrides chemically store hydrogen at least partly within their bulk within interstitial sites between metal atoms.
Carbon nanotubes may have other drawbacks. They are expensive to manufacture and especially to purify. They could potentially create an explosion hazard in an oxygen environment. Also, they usually need to be kept at low temperatures in order to retain hydrogen.
Current hydrogen storage technology focuses on macroscale hydrides, carbon nanotubes, compressed hydrogen and cryogenic liquid hydrogen. Instead of just improving these existing systems, it would be beneficial to take an entirely different approach.